ALK-2, also known as activin A receptor, type I (ACVR1) or as serine threonine protein kinase receptor R1 (SKR1) is a protein kinase which in humans is encoded by the ACVR1 gene.
ALK-2 is a type I BMP receptor which is widely expressed. It comprises an extracellular ligand binding domain and a regulated intracellular serine/threonine kinase domain, both required for signal transduction.
Bone morphogenic proteins (BMPs) are multi-functional growth factors that are members of the transforming growth factor β (TGFβ) superfamily. BMP signaling plays a role in heart, neural, and cartilage development as well as in postnatal bone formation. BMPs ectopically induce endochondral bone formation and play a critical role in skeletal and joint morphogenesis (Urist, Science 110:893-899 (1965); Olsen et al, Annu. Rev. Cell Dev. Biol. 16:191-220 (2000); Kronenberg, Nature 423:332-336 (2003); Thomas et al, Nat. Genet. 12:315-317 (1996); Thomas et al, Nat. Genet. 17:58-64 (1997); Polinkowsky et al, Nat. Genet. 17:18-19 (1997); Storm et al., Nature 368:639-643 (1994); and Wozney, Prog. Growth Factor Res. 1:267-280 (1989)).
BMP signaling is controlled at many levels, including via extracellular antagonists such as noggin (Massague, Nat. Rev. Mol. Cell. Biol. 1:169-178 (2000)). It has been suggested that untimely or unwanted activation of signaling pathways fundamental for normal development may promote disease processes such as spondyloarthropathies. The effects of BMP signaling on initiation and progression of arthritis by gene transfer of noggin have also been described (Lories et al, J. Clin. Invest., 115, 1571-1579 (2005)). The physiological roles of BMPs and BMP receptor signaling in normal bone formation, including skeletal and limb development, have been studied and reviewed in Zhao, Genetics 35:43-56 (2003).
Experiments with BMP antagonists demonstrate that regulation of BMP signaling proteins is central to bone formation in vivo (Devlin et al., Endocrinology 144:1972-1978 (2003) and Wu et al., J. Clin. Invest., 112: 924 (2003)).
Fibrodysplasia ossificans progressiva (FOP) is a rare and disabling genetic disorder characterized by congenital malformations of the great toes and by progressive heterotopic endonchodral ossification in predictable anatomical patterns. Ectopic expression of BMP4 has been found in FOP patients (Gannon et al., Hum. Pathol. 28:339-343 (1997) and Xu et al, Clin. Genet. 58:291-298 (2000)). It has been shown that patients with FOP have activating mutations in ALK-2 (Shore et al., Nat. Genet., 38(5):525-7 (2006)).
It has been established that excessive BMP signaling leads to a number of conditions described above. WO2008033408 and WO2009114180 describe inhibitors of the BMP signaling pathway. There is still however a constant need to find alternative ways in which BMP signaling can be regulated.
Such a need can be met by designing selective ALK-2 inhibitors.
Specific ALK-2 antibodies are described for instance in WO1994011502 and WO2008030611. Osteogenic proteins that bind to ALK-2 are described in WO2012023113 and WO2012077031.
WO2007123896 describes a method of treating a pathology associated with heterotopic ossification by administering siRNA specific against a nucleic acid encoding a mutated ALK-2.
WO2014160203 and WO2014138088 describe inhibitors of the BMP pathway. WO 2015152183 describes ALK-2 inhibitors for the treatment of FOP. Inhibitors of ALK-2 are also described in WO2014151871.